Crowd Scene: Generating many characters with random clothing objects
If you're great at scripting, one way might be to use the shape keys. If you instance the objects and randomize the shape keys for different body heights and shapes and randomize the texture indexes, that probably should do it. With shape keys you can completely do a seamless remodel, so it might be good to start with... for example long sleeve, long hair- then model all other changes into shape keys.

It can do a final scene check in the strange event that two bodies match by checking first if any two parameters are the same, then continuing the check until it is negative. Switching a parameter if all parameters match two objects.